Radon is recognized as a powerful tracer of certain geophysical processes in marine and aquatic environments. In the past few decades, the instruments and methods for measuring radon concentration in water have been developed to some extent but still lack underwater in-situ measurements. Here we present an in-situ detection equipment for radon-in-water (pulsed ionization chamber (PIC)-radon) to measure dissolved radon in ocean and groundwater settings. The equipment has been successfully deployed in the Jiaozhou Bay in July 2022 and has achieved 14 d of unattended underwater in-situ observation. Then it was successfully placed in a groundwater monitoring well in the Laizhou Bay in November 2022 and monitored radon activities for over 30 d. The results showed that this instrument had a good indication of submarine groundwater discharge. The PIC-radon detector takes advantage of smaller size, lower power consumption, and is barely influenced by humidity, making it particularly suitable for long-term in-situ measurement, especially in harsh environments with limited human care or deployment spaces.

Subterranean estuaries, i.e., the mixing zone between terrestrial groundwater and recirculated seawater, host a wide range of microbiota. Here, field campaigns were conducted at the mouth of the subterranean estuary at the Sanggou Bay (Shandong Province, China) over four consecutive seasons at a seepage face (0−20 cm depth). The diversity of benthic microbiome was characterized via 16S rRNA gene sequencing and metagenomics, combined with physic-chemical parameters, e.g., organic carbon, total nitrogen and sulfate contents in sediments. During spring, the dominant species were assigned to the phylum Proteobacteria. Important opportunistic species was assigned to Acidobacteria, Actinobacteria and Bacteroidetes. The key components were identified to be species of the genera Pseudoalteromonas, Colwellia and Sphingobium, indicating the involvement of sediment microbiota in the degradation of sedimentary organic carbon, particularly that of pelagic origin, e.g., phytoplankton detritus and bivalve pseudo-feces. During spring, the microbial community was statistically similar along the depth profiles and among the three sampled stations. Similar spatial distributions were obtained in the remaining seasons. By contrast, the dominant species assemblages varied significantly among seasons, with key genera being Thioprofundum and Nitrosopumilus during summer and autumn and Thioprofundum and Ilumatobacter during winter. Network analysis revealed a seasonal shift in benthic nitrogen and sulfur metabolism associated with these variations in microbial community composition. Overall, our findings suggested that macro elements derived from pelagic inputs, particularly detrital phytoplankton, shaped the microbial community compositions at the seepage face, resulting in significant seasonal variations, while the influence of terrestrial materials transported by groundwater on the sediment microbiota at the seepage face found to be minor.

The horizontal structure of mangrove forests is an important characteristic that reflects a significant signal for coupling between mangroves and external drivers. While the loss and gain of mangroves has received much attention, little information about how the horizontal structure of mangrove forests develops from the seedling stage to maturity has been presented. Here, remote sensing images taken over approximately 15 years, UVA images, nutrient elements, sediments, and Aegiceras corniculatum vegetation parameters of the ecological quadrats along the Nanliu Delta, the largest delta of the northern Beibu Gulf in China, are analyzed to reveal changes in the horizontal structure of mangroves and their associated driving factors. The results show that both discrete structures and agglomerated structures can often be found in A. corniculatum seedlings and saplings. However, the combination of seedlings growing into maturity and new seedlings filling in available gaps causes the discrete structure of A. corniculatum to gradually vanish and the agglomerate structure to become stable. The aggregated structure of seedlings, compared to the discrete structure, can enhance the elevation beneath mangroves by trapping significantly more sediments, providing available spaces and conditions for seedlings to continue growing. Furthermore, by catching fine sediments with enriched nutrients, the survival rate of A. corniculatum seedlings in the agglomerated structure can be much higher than that in the discrete structure. Our results highlight the significance of the agglomeration of A. corniculatum, which can be beneficial to coastal mangrove restoration and management.

A high-frequency, high-resolution shore-based video monitoring system (VMS) was installed on a macrotidal (tidal amplitude >4 m) beach with multiple cusps along the Quanzhou coast, China. Herein, we propose a video imagery-based method that is coupled with waterline and water level observations to reconstruct the terrain of the intertidal zone over one tidal cycle. Furthermore, the beach cusp system (BCS) was precisely processed and embedded into the digital elevation model (DEM) to more effectively express the microrelief and detailed characteristics of the intertidal zone. During a field experiment conducted in January 2022, the reconstructed DEM was deemed satisfactory. The DEM was verified by RTK-GPS and had an average vertical root mean square error along corresponding RTK-GPS-derived intertidal profiles and corresponding BCS points of 0.134 m and 0.065 m, respectively. The results suggest that VMSs are an effective tool for investigating coastal geomorphic processes.

Mangroves are crucial for protecting coastal areas against extreme disasters such as tsunamis and storm surges. An experimental study was conducted to determine how mangroves can mitigate the tsunami wave propagation. The test was performed in a flume, where mangrove models were installed on a slope, and dam-burst waves were used to simulate tsunami waves. To study how mangrove forests reduce the impact of tsunamis, this paper measured the heights of the incoming waves under different initial conditions (tsunami wave intensity and initial water depth) and plant factors (arrangement and distribution density) and described the reduction process. The results show that, after passing through the mangrove, the tsunami bore height will decrease within a certain range as the initial water depth increases. However, there is no correlation between the increase of inundation level and the drop of water level. The bore height attenuation is more significant at higher density of mangroves, but after tsunami passing through the mangroves, the relative bore height will decrease. When the distribution density of mangroves is constant, the wave attenuation at different locations (before, on and after the slope) shows different relationships with the initial water depth and wave height for different models. The transmission coefficient ($ {K }_{i} $) shows a parabolic correlation with its density. The proportion of the energy loss caused by the mangrove resistance to the total energy ($ {E }_{b} $) is defined as $ {C}_{m2} $. The variation trend of $ {C}_{m2} $ corresponds to the tsunami wave energy attenuation rate ($ {C}_{a} $) and $ {K }_{i} $.

Low tide terrace beach is a main beach type along South China coasts with strong tidal actions. How strong tides affect wave transformations on low tide terrace beach still remains unclear. In this study, in-situ measurements are conducted on the low terrace beach at Xisha Bay to provide quantitative descriptions of wave shoaling and shore-breaker phenomena under the tidal effects. It is found that wave breaking is unsaturated on the low tide terrace beach at Xisha Bay. Magnitudes of wave skewness and asymmetry increase as wave shoals and achieve the maximum value at the shore-breaker, and then decrease rapidly. Mean energy dissipation rates of shore-breakers are tide-modulated since the bottom slope changes at the shoreward boundary of wave propagation in a tidal cycle. The remaining wave energy flux at the initialization of the shore-breaker is 1%–12% of offshore wave energy flux, and the energy flux ratio decreases with increasing offshore wave heights. Wave attenuation at shore-breakers can be estimated directly from offshore wave conditions based on findings in this study, favoring designs of seawalls or beach nourishment projects. Field datasets on wave transformations can also be used for verifications of wave numerical models.

The clay mineralogy of 28 sandy-muddy transitional beach (SMT-Beach) sediments and surrounding mountain river sediments along the coasts of southeastern China was systematically investigated to reveal the sediment source-to-sink process variations of such beaches and their morphological indications. The results show that the clay mineral assemblages of these SMT-Beaches mainly comprise of almost equal illite (~30%), kaolinite (~28%), chlorite (~22%), and smectite (~20%) contents. From the surrounding mountain rivers to the SMT-Beaches, clay mineral assemblages show distinct spatial changes characterized by a large decrease (~40%) in kaolinite, whereas the other three clay minerals present relative increases, especially clear for smectite. The muddy sediment sources of SMT-Beaches inferred from the clay mineralogy are mainly derived from nearby mountain rivers coupled with long-distance transport and penetration of the Changjiang River. The sandy sediments of these beaches are predominantly sourced from nearby mountain rivers, the weathering products of surrounding rocks in both mainland and island environments, and erosion of the “Old Red Sand” and “Red Soil Platform”. However, the sandy sediment sources of the SMT-Beaches are largely reduced because of the remarkable decrease in the river fluvial supply associated with intensive human activities such as dam construction and coastal reclamation. Subsequently, the sandy sections of SMT-Beaches present clear erosion and have revealed by both time series remote sensing images and a compilation of published literature. In contrast, the muddy sediment supply of SMT-Beaches is temporarily stable and relatively constant, resulting in the landward migration of the mudflats with relative transgression or accumulation. These findings highlight that the natural evolution processes of SMT-Beaches have been greatly reshaped by intensive human activities.

The change of coastal wetland vulnerability affects the ecological environment and the economic development of the estuary area. In the past, most of the assessment studies on the vulnerability of coastal ecosystems stayed in static qualitative research, lacking predictability, and the qualitative and quantitative relationship was not objective enough. In this study, the “Source-Pathway-Receptor-Consequence” model and the Intergovernmental Panel on Climate Change vulnerability definition were used to analyze the main impact of sea level rise caused by climate change on coastal wetland ecosystem in Minjiang River Estuary. The results show that: (1) With the increase of time and carbon emission, the area of high vulnerability and the higher vulnerability increased continuously, and the area of low vulnerability and the lower vulnerability decreased. (2) The eastern and northeastern part of the Culu Island in the Minjiang River Estuary of Fujian Province and the eastern coastal wetland of Meihua Town in Changle District are areas with high vulnerability risk. The area of high vulnerability area of coastal wetland under high emission scenario is wider than that under low emission scenario. (3) Under different sea level rise scenarios, elevation has the greatest impact on the vulnerability of coastal wetlands, and slope has less impact. The impact of sea level rise caused by climate change on the coastal wetland ecosystem in the Minjiang River Estuary is mainly manifested in the sea level rise, which changes the habitat elevation and daily flooding time of coastal wetlands, and then affects the survival and distribution of coastal wetland ecosystems.

The cross-shore variation in wind speeds influenced by beach nourishment, especially the dramatic changes at the nourished berm, is important for understanding the aeolian sand transport processes that occur after beach nourishment, which will contribute to better beach nourishment project design on windy coasts. In this paper, the influencing factors and potential mechanism of wind speed variation at the edge of a nourished berm were studied. Field observations, together with the Duna model, were used to study the cross-shore wind speed distribution for different nourishment schemes. The results show that the nourished berm elevation and beachface slope are the main factors controlling the increase in wind speed at the berm edge. When the upper beach slope is constant, the wind speed at the berm edge has a positive linear correlation with the berm elevation. When the berm elevation remains constant, the wind speed at the berm edge is also proportional to the upper beach slope. Considering the coupling effects of nourished berm elevation and beachface slope, a model for predicting the wind speed amplification rate at the nourished berm edge was established, and the underlying coupling mechanism was illustrated.

Mangrove forest is one of the most important ecological and environmental resources by effectively promoting tidal flat deposition and preventing the coastal region from typhoon. However, there have been mass loss of mangrove forests due to anthropogenic activities. It is an urgent need to explore an effective way for mangrove restoration. Here, three rows of bamboo fences with hydro-sedimentary observation set over Aegiceras corniculatum mangrove tidal flat of the Nanliu Delta, the largest delta of Beibu Gulf, China, were conducted to analyze the hydro-sedimentary variations induced by bamboo fences. Results identified that the mean horizontal velocity U_m per burst (20 min) decreased by as much as 71% and 40% in comparison with those without bamboo fences in March and November, respectively, when the tidal current entering the bamboo area during flood. The maximum of mean horizontal flow velocity U_m-max at bamboo area was 50%–75% of that without bamboo fences during ebb tide. The suspended sediment concentration of bamboo area suggested a maximum reduction of 57% relative to bare flat during flood, and was 80% lower than bare flat at ebb peak. Moreover, the turbulent kinetic dissipation ε at flood tide was significantly higher than that at ebb tide, while the bamboo fences greatly increased the turbulent kinetic dissipation ε by 2 to 5 times relative to bare flat, resulting in an increase of the bed elevation by inhibiting the sediment incipient motion and intercepting suspended sediment. The siltation rate at the bamboo area was 140% and 29.3% higher than that at the bare flat and the region covered with A. corniculatum, respectively. These results highlight that bamboo fences can effectively attenuate tidal current and thus promote siltation over mangrove flat, which contribute great benefit to mangrove survival.